52 zyxwvutsrqpo LOW COMPLEXITY ADAPTIVE RECEIVER FOR CDMA WITH MULTIPATH FADING Sridhar Gollamudi, Shirish Nagaraj, Samir Kapoor and Yih-Fang Huang Laboratory for Image and Signal Analysis Department of Electrical Engineering University of Notre Dame Notre Dame, IN 46556. USA. E-mail: huang.2Qnd.edu zyxw ABSTRACT A new receiver scheme for a DS-CDMA system similar to the IS-95 uplink is proposed in this paper that is of lower complexity and superior bit error rate perfor- m-ance in comparison to the conventional non-coherent Rake receiver. The proposed receiver consists of a co- herent Rake receiver in the form of a transversal filter, and uses channel estimates obtained by an adaptive channel identification algorithm. Adaptation follows the zyxwvutsrqpo Set-Membership Filtering methodology, that fea- tures fast convergence and lower computational com- plexity compared to least-squares techniques. I. INTRODUCTION There has been a tremendous research interest in recent years in adaptive receivers for CDMA systems that counter the effects of multipath fading and mul- tiaccess interference (MAI). However, almost all the proposed techniques rest on two critical assumptions, namely, that the modulation is linear (e.g., BPSK) and that the spreading codes have a time period equal to the bit interval. Both these assumptions are not valid in typical multiaccess cellular CDMA systems such as the uplink in the IS-95 standard. This paper proposes an adaptive scheme that improves the performance of a CDMA system which employs M-ary orthogonal modulation and spreading sequences with no assumed (or large) periodicity, zyxwvuts as is the case in IS-95. The Rake receiver [I] is commonly used to provide time-diversity in a multipath environment by combin- ing multiple copies of the transmitted signal that reach the receiver at different delays. Since it is wasteful of This work was supported, in part, by the National Science Foundation under Grant MIP 97-05173, in part, by Lucent Tech- nologies, Whippany, NJ, USA, in part, by the Center for Applied Mathematics, University of Notre Dame, and in part, by the Tellabs Research Center, Mishawaka, IN, USA. 0-7803-4298-4/97/$10.000 1997 EEE power for the mobile transmitters to transmit pilot signals to aid phase recovery at the receiver, Rake combining and demodulation are usually carried out non-coherently. The reader is referred to [2] and the references therein for details on DS-CDMA reception. Performance of a Rake receiver can be improved if the Rake fingers are combined and demodulated coherently, with the help of phase estimates on all the paths. Further improvement is possible via maximal ratio combining, that is, by weighting the Rake fingers in proportion to their signal strengths to maximize the output signal-to-noise ratio. Both these techniques can be incorporated if accurate estimates of complex channel gains are obtained. As will be shown later in this paper, the resulting Rake receiver is simply a matched filter that is matched to the multipath channel. Receiver complexity is substantially reduced due to the fact that the proposed receiver requires a single despreader and a single set of M-ary symbol correlators per each user, as opposed to one for each finger of the Rake in conventional receivers. Complex channel estimates can be obtained by an adaptive identification algorithm with the help of training signals. A novel and emerging adaptive filtering paradigm known as Set-Membership Filtering (SMF) is utilized in this paper for this purpose. SMF is an extension of the well-studied Set-Membership z Identification (SMI) methodology [3, 4, zy 5, zyx 6, 7, 81 for bounded-noise systems to the general parameter estimation problem for a linear-in-parameters model [9, 10, 111. SMF algorithms offer several advantages over stochastic gradient algorithms such as the LMS algorithm and least-squares techniques such as the RLS algorithm in terms of convergence behavior, tracking characteristics and computational complexity. This paper is organized as follows. The CDMA sys- tem model is introduced in the following section. Sec- ICPWC’97